Conventionally, vibrations caused by a knocking phenomenon generated in an internal combustion engine are detected as knocking detection signals by a knocking sensor,so that the knocking detection signals are allowed to pass through a filter circuit to remove noise components of low frequency and high frequency and only the knocking frequency component of specific frequency is allowed to pass, and thereafter, only the detected signal of a predetermined period is taken out to thereby compare a magnitude of wave form of the detected signal with a previously preset set value for decision, so as to decide whether the knocking exists or not, which is well-known as disclosed in, for example, the Japanese Patent Publication Gazette No. Hei 7-13507.
Conventionally, one vibration detector is used for detecting knocking vibrations from a plurality of cylinders, for example, three to four cylinders, and distances from respective cylinders to the vibration detector are different, so that even when the respective cylinders are equal in strength generated therein, the magnitudes of signal detected by the vibration detector are different from each other, whereby there has been the inconvenience that, regardless of generating the knocking, the signals detected by the vibration detector lead to decision of no knocking generation.
An extent of damage given to the engine, when the knocking is generated, changes according to the strength of knocking and frequency of knocking generation, whereby the ignition timing control, which is performed only by deciding the existence of knocking as conventional, is insufficient for the knocking avoid control, resulting in that the knocking cannot be avoided in spite of controlling.